Recent Progress in Two-Dimensional Materials for Electrocatalytic CO2 Reduction

Electrocatalytic CO2 reduction (ECR) is an attractive approach to convert atmospheric CO2 to value-added chemicals and fuels. However, this process is still hindered by sluggish CO2 reaction kinetics and the lack of efficient electrocatalysts. Therefore, new strategies for electrocatalyst design should be developed to solve these problems. Two-dimensional (2D) materials possess great potential in ECR because of their unique electronic and structural properties, excellent electrical conductivity, high atomic utilization and high specific surface area. In this review, we summarize the recent progress on 2D electrocatalysts applied in ECR. We first give a brief description of ECR fundamentals and then discuss in detail the development of different types of 2D electrocatalysts for ECR, including metal, graphene-based materials, transition metal dichalcogenides (TMDs), metal–organic frameworks (MOFs), metal oxide nanosheets and 2D materials incorporated with single atoms as single-atom catalysts (SACs). Metals, such as Ag, Cu, Au, Pt and Pd, graphene-based materials, metal-doped nitric carbide, TMDs and MOFs can mostly only produce CO with a Faradic efficiencies (FE) of 80~90%. Particularly, SACs can exhibit FEs of CO higher than 90%. Metal oxides and graphene-based materials can produce HCOOH, but the FEs are generally lower than that of CO. Only Cu-based materials can produce high carbon products such as C2H4 but they have low product selectivity. It was proposed that the design and synthesis of novel 2D materials for ECR should be based on thorough understanding of the reaction mechanism through combined theoretical prediction with experimental study, especially in situ characterization techniques. The gap between laboratory synthesis and large-scale production of 2D materials also needs to be closed for commercial applications.publishedVersio

Leveraging Synergies by Combining Polytetrafluorethylene with Polyvinylidene Fluoride for Solvent-Free Graphite Anode Fabrication

Solvent-free graphite anode is fabricated successfully with the synergistic effect of polytetrafluorethylene (PTFE) and polyvinylidene fluoride (PVDF). PTFE acts as a processing aid reagent to form a self-supporting electrode film, while PVDF acts as a functional binder when PTFE decomposes in the first lithiation process. The solvent-free graphite electrode with high loading of 15 mg cm−2 shows good stability with more than 95% capacity retention after 50 charge/discharge cycles under the current of 0.23 mA cm−2. Electrodes with extra high loading of 27 mg cm−2 (8.2 mAh cm−2) are fabricated and show good stability. Initial coulombic efficiency increases to 89% after prelithiation in the full cell with lithium iron phosphate as cathode. The capacity retention of full cells is more than 80% after 110 cycles under the current of 0.7 mA cm−2 in coin cells. The roll-to-roll production makes the procedure compatible with current commercial lithium-ion batteries production lines, exhibiting great potential for upscaling production.publishedVersio

Unravelling the electrochemical impedance spectroscopy of silicon half cells with commercial loading

Silicon (Si) is an important anode material for lithium ion batteries (LIBs), and increasing the loading of Si electrodes is an important step towards commercialization. However, half cells commonly used for Si studies are limited by polarization of the lithium (Li) counter electrode, especially at high Si loading. To study the interplay between Si and Li electrodes, a set of electrochemical impedance spectroscopy (EIS) spectra are generated using cycled Si half cells at four different potentials in the charge–discharge profile, and then repeated using symmetric Si/Si and Li/Li cells assembled from half cells cycled to equivalent stages in the cycle. Distribution of relaxation times (DRT) analysis is used to design equivalent circuits (ECs) for both Si/Si and Li/Li symmetric cells incorporating both electrolyte and electrode-related diffusion, and these are applied to the half cells. The results demonstrate that the behaviour of half cells is dominated by the solid electrolyte interphase (SEI) impedances at the Li counter electrode at the low and high potentials where the Li+ mobility signal in Si is limited, while the Si electrode is dominant at intermediate potentials where the signal from mobile Li+ is strong. EIS studies of Si half cells should therefore be performed at intermediate potentials, or as symmetric cells.publishedVersio

Solvent-free lithium iron phosphate cathode fabrication with fibrillation of polytetrafluoroethylene

Fabricating electrode for lithium-ion batteries (LiBs) with solvent-free (SF) procedure can save energy and improve electrochemical performance simultaneously. Polymer fibrillation is one of the most promising SF procedures due to its feasibility for upscale production. The hardness of lithium iron phosphate (LFP) impedes its SF fabrication with polytetrafluoroethylene (PTFE) fibrillation. In this study, we successfully expanded PTFE fibrillation for SF LFP electrode fabrication with the help of carbon nanotubes (CNTs). CNTs increase the conductivity of electrode, and act as matrix for LFP particles to ensure relative displacement to further fibrillate PTFE to form self-supporting electrode film when the dry mixture was hot rolled. The SF LFP/hard carbon full cells were fabricated and demonstrated comparable electrochemical performance to slurry casting (SC) fabricated LFP electrode. The initial coulombic efficiency (ICE) of full cell increased to more than 95% after prelithiation.publishedVersio

Sulfur‐Decorated Ni−N−C Catalyst for Electrocatalytic CO2 Reduction with Near 100 % CO Selectivity

Developing highly efficient electrocatalysts for electrochemical CO2 reduction (ECR) to value-added products is important for CO2 conversion and utilization technologies. In this work, a sulfur-doped Ni−N−C catalyst is fabricated through a facile ion-adsorption and pyrolysis treatment. The resulting Ni−NS−C catalyst exhibits higher activity in ECR to CO than S-free Ni−N−C, yielding a current density of 20.5 mA cm−2 under −0.80 V versus a reversible hydrogen electrode (vs. RHE) and a maximum CO faradaic efficiency of nearly 100 %. It also displays excellent stability with negligible activity decay after electrocatalysis for 19 h. A combination of experimental investigations and DFT calculations demonstrates that the high activity and selectivity of ECR to CO is due to a synergistic effect of the S and Ni−NX moieties. This work provides insights for the design and synthesis of nonmetal atom-decorated M−N−C-based ECR electrocatalysts.publishedVersio

Enabling Increased Delithiation Rates in Silicon-Based Anodes through Alloying with Phosphorus

The capability of battery materials to deliver not only high lithium storage capacity, but also the ability to operate at high charge/discharge rates is an essential property for development of new batteries. In the present work, the influence on the charge/discharge rate behaviour of substoichiometric concentrations of phosphorus (P) in silicon (Si) nanoparticles was studied. The results revealed an increase in rate capability as a function of the P concentration between 0 and 5.2 at %, particularly during delithiation. The stoichiometry of the nanoparticles was found to strongly affect the formation of the Li3.5Si phase during lithiation. Cyclic stability experiments demonstrated an initial increase in capacity for the SiPx materials. Galvanostatic intermittent titration technique and electrochemical impedance spectroscopy demonstrated the increased lithium diffusivity with inclusion of P. Density functional theory and ab initio molecular dynamics were deployed to provide a rationale for the electrochemical behaviour of SiPx.publishedVersio

Enabling Increased Delithiation Rates in Silicon-Based Anodes through Alloying with Phosphorus

The capability of battery materials to deliver not only high lithium storage capacity, but also the ability to operate at high charge/discharge rates is an essential property for development of new batteries. In the present work, the influence on the charge/discharge rate behaviour of substoichiometric concentrations of phosphorus (P) in silicon (Si) nanoparticles was studied. The results revealed an increase in rate capability as a function of the P concentration between 0 and 5.2 at %, particularly during delithiation. The stoichiometry of the nanoparticles was found to strongly affect the formation of the Li3.5Si phase during lithiation. Cyclic stability experiments demonstrated an initial increase in capacity for the SiPx materials. Galvanostatic intermittent titration technique and electrochemical impedance spectroscopy demonstrated the increased lithium diffusivity with inclusion of P. Density functional theory and ab initio molecular dynamics were deployed to provide a rationale for the electrochemical behaviour of SiPx.publishedVersio

Precise Measurements of Branching Fractions for Ds+D_s^+ Meson Decays to Two Pseudoscalar Mesons

We measure the branching fractions for seven $D_{s}^{+}$ two-body decays to pseudo-scalar mesons, by analyzing data collected at $\sqrt{s}=4.178\sim4.226$ GeV with the BESIII detector at the BEPCII collider. The branching fractions are determined to be $\mathcal{B}(D_s^+\to K^+\eta^{\prime})=(2.68\pm0.17\pm0.17\pm0.08)\times10^{-3}$, $\mathcal{B}(D_s^+\to\eta^{\prime}\pi^+)=(37.8\pm0.4\pm2.1\pm1.2)\times10^{-3}$, $\mathcal{B}(D_s^+\to K^+\eta)=(1.62\pm0.10\pm0.03\pm0.05)\times10^{-3}$, $\mathcal{B}(D_s^+\to\eta\pi^+)=(17.41\pm0.18\pm0.27\pm0.54)\times10^{-3}$, $\mathcal{B}(D_s^+\to K^+K_S^0)=(15.02\pm0.10\pm0.27\pm0.47)\times10^{-3}$, $\mathcal{B}(D_s^+\to K_S^0\pi^+)=(1.109\pm0.034\pm0.023\pm0.035)\times10^{-3}$, $\mathcal{B}(D_s^+\to K^+\pi^0)=(0.748\pm0.049\pm0.018\pm0.023)\times10^{-3}$, where the first uncertainties are statistical, the second are systematic, and the third are from external input branching fraction of the normalization mode $D_s^+\to K^+K^-\pi^+$. Precision of our measurements is significantly improved compared with that of the current world average values

On the Viability of Lithium Bis(fluorosulfonyl)imide as Electrolyte Salt for Use in Lithium-Ion Capacitors

Lithium-ion capacitors (LICs) represent promising high-power energy storage devices, most commonly composed of a lithium-ion intercalation anode (e. g., graphite or hard carbon), a supercapacitor activated carbon (AC) cathode, and an electrolyte with 1 M LiPF6 in carbonate solvents. LiPF6 is susceptible to hydrolysis, forming HF, which leads to challenges for disassembly and recycling, risks during hazardous events, and extensive energy consumption during production. Here, we report on the feasibility of replacing LiPF6 with the non-hydrolysing salt LiFSI for use with AC electrodes. Based on voltage hold measurements in a half-cell setup, good long-term stability is achieved with an upper cut-off voltage of 3.95 V vs. Li/Li+, potentially enabling cell voltages of ~3.8 V when combined with graphite or silicon-based anodes (operating at ~0.1 V vs. Li/Li+) in LIC full cells. The lower cut-off voltage was determined to be 2.15 V vs. Li/Li+. The systematic comparison of CV, leakage current analysis and capacity retention upon voltage hold highlights the importance of the latter method to provide a realistic assessment of the electrochemical stability window (ESW) of LiFSI on a commercial AC electrode. The morphological and surface-chemical post-mortem analysis of AC electrodes used with LiFSI revealed that the oxidation of the FSI anion, as evidenced by the presence of new S 2p and N 1s features in the XPS spectra, and an increasing number of oxygenated species on the AC were the main processes causing capacity fade at positive polarization.publishedVersio

Transition Metal−N/Graphene for advanced Lithium–Sulfur Batteries: A first principles study

Developing highly efficient anchoring materials with strong adsorption to lithium polysulfides to suppress ‘shuttling effect’ is crucial to address the short cycling life issue of lithium sulfur batteries. Herein, we systematically investigated transition metal, nitrogen co-doped graphene as host materials for sulfur cathode via first-principles study. The computation results reveal that TM2–Nsingle bondC materials have higher adsorption energy to lithium polysulfides than TM–Nsingle bondC materials. Fe2–Nsingle bondC is one of the most promising anchoring materials, where robust Fe–S and N–Li bonds ensure the stable adsorption of all LiPSs.publishedVersio